chore: upgrade syn to 3.0.3

[tisonkun]

Summary

• upgrade the fastrace-macro direct dependency from syn 2.0.118 to 3.0.3
• adapt function signature handling to the syn 3 Safety API
• initialize the new TypeInfer attrs field while preserving the original underscore span

Testing

• cargo +nightly fmt --all -- --check
• cargo +nightly clippy --all-targets --all-features -- --deny warnings
• cargo +nightly clippy in examples and benches
• cargo build --workspace --all-features --all-targets
• cargo nextest run --workspace (67 passed)
• cargo test --doc

[tisonkun]

@andylokandy I'm going to cut a new release on this dep upgrade.